In recent years, with increased demand for speeding up in a radio communication system, an attempt has been made to achieve a multilevel modulation and demodulation or a multicarrier modulation and demodulation. However, because the frequency allocation used in a radio communication system is limited, there is a limitation in attempting the multicarrier modulation and demodulation. Thus, a radio communication system using a Multiple Input Multiple Output (MIMO) communication method has been developed. According to the MIMO communication method, transmission data is previously divided into a plurality of signals (streams), and the divided signals are simultaneously transmitted through a plurality of antennae in the same frequency band.
FIG. 1 illustrates a conventional radio communication system 10 using a MINO communication method (refer to Patent Document 1).
A transmitter station 11 includes a transmission signal processing part (not illustrated in the figure), a modulating part (not illustrated in the figure), a transmission amplifier (not illustrated in the figure), and a plurality of antennas 12A and 12B. Transmission data is divided into a plurality of signals in the transmission signal processing part. The divided signals are modulated by the modulating part, and are amplified by the transmission amplifier to have a desired power. Then, the amplified signals are transmitted through the antennas 12A and 12B.
A receiver station 15 includes a receiver amplifier (not illustrated in the figure), a demodulating part (not illustrated in the figure), a reception signal processing part (not illustrated in the figure) and a plurality of antennas 16A and 16B. Signals received by the antennae 16A and 16B are amplified by the reception amplifier, and demodulated by the demodulating part to be reception data. The thus-processed reception data is subjected to a separation process using a channel response matrix in the reception signal processing part.
A transmission rate according to the MIMO communication method is proportional to the number of antennas. Thus, it is possible to increase the transmission rate while using the same frequency and the same time.
Patent Document 1: Japanese Laid-Open Patent Application No. 2006-295433
According to MIMO communication method, if a multipath signals come from a large angle range and if there is an antenna interval equal to or greater than a half-wavelength, the elements of the channel response matrix vary without correlation. Under such a condition, the transmission capacity of the radio communication system is increased.
However, there is a problem in that a transmission capacity is small in a case where there is no local scattering in a line-of-sight communication because the elements of the channel response matrix do not vary without correlation unless the antenna interval is large.